SynchronizedQueue.java

/*
 * Copyright (C) 2009 eXo Platform SAS.
 *
 * This is free software; you can redistribute it and/or modify it
 * under the terms of the GNU Lesser General Public License as
 * published by the Free Software Foundation; either version 2.1 of
 * the License, or (at your option) any later version.
 *
 * This software is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with this software; if not, write to the Free
 * Software Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA
 * 02110-1301 USA, or see the FSF site: http://www.fsf.org.
 */
package org.exoplatform.services.cache.concurrent;

import org.exoplatform.services.log.Log;

import java.util.ArrayList;
import java.util.concurrent.atomic.AtomicBoolean;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

/**
 * @author <a href="mailto:julien.viet@exoplatform.com">Julien Viet</a>
 * @version $Revision$
 */
public class SynchronizedQueue<I extends Item> implements Queue<I>
{

   private final Log log;

   private final Item head;

   private final Item tail;

   volatile int queueSize; // The queue size cached (which can be an estimate)

   private final Lock queueLock = new ReentrantLock();

   private volatile AtomicBoolean trimming = new AtomicBoolean();

   public SynchronizedQueue(Log log)
   {
      this.log = log;
      this.head = new Item();
      this.tail = new Item();
      this.queueSize = 0;

      //
      head.next = tail;
      tail.previous = head;

      //
      if (isTraceEnabled())
      {
         trace("Queue initialized with first=" + head.serial + " and last=" + tail.serial);
      }
   }

   private boolean isTraceEnabled()
   {
      return log != null && log.isTraceEnabled();
   }

   private void trace(String message)
   {
      log.trace(message + " [" + Thread.currentThread().getName() + "]");
   }

   public int size()
   {
      return queueSize;
   }

   public void assertConsistency()
   {
      int cachedQueueSize = queueSize;
      int effectiveQueueSize = 0;
      for (Item item = head.next; item != tail; item = item.next) //NOSONAR
      {
         effectiveQueueSize++;
      }

      if (effectiveQueueSize != cachedQueueSize)
      {
         throw new AssertionError("The cached queue size " + cachedQueueSize
            + "  is different from the effective queue size" + effectiveQueueSize);
      }
   }

   /**
    * Attempt to remove an item from the queue.
    *
    * @param item the item to remove
    * @return true if the item was removed by this thread
    */
   public boolean remove(I item)
   {
      boolean trace = isTraceEnabled();
      queueLock.lock();
      try
      {
         Item previous = item.previous;
         Item next = item.next;
         if (previous != null && next != null)
         {
            previous.next = next;
            next.previous = previous;
            item.previous = null;
            item.next = null;
            int newSize = --queueSize;
            if (trace)
            {
               trace("Removed item=" + item.serial + " with previous=" + previous.serial + " and next=" + next.serial
                  + " with queue=" + newSize + "");
            }
            return true;
         }
         else
         {
            if (trace)
            {
               trace("Attempt to remove item=" + item.serial + " concurrently removed");
            }
            return false;
         }
      }
      finally
      {
         queueLock.unlock();
      }
   }

   /**
    * Add the item to the head of the list.
    *
    * @param item the item to add
    */
   public void add(I item)
   {
      queueLock.lock();
      try
      {
         Item next = head.next;
         item.next = next;
         next.previous = item;
         head.next = item;
         item.previous = head;
         int newSize = ++queueSize;
         if (isTraceEnabled())
         {
            trace("Added item=" + item.serial + " with next=" + next.serial + " and queue=" + newSize);
         }
      }
      finally
      {
         queueLock.unlock();
      }
   }

   /**
    * Attempt to trim the queue. Trim will occur if no other thread is already performing a trim
    * and the queue size is greater than the provided size.
    *
    * @param size the wanted size
    * @return the list of evicted items
    */
   public ArrayList<I> trim(int size)
   {
      if (trimming.compareAndSet(false, true))
      {
         try
         {
            queueLock.lock();
            try
            {
               if (queueSize > size)
               {
                  ArrayList<I> evictedItems = new ArrayList<I>(queueSize - size);
                  while (queueSize > size)
                  {
                     I last = (I)tail.previous;
                     remove(last);
                     evictedItems.add(last);
                  }
                  return evictedItems;
               }
            }
            finally
            {
               queueLock.unlock();
            }
         }
         finally
         {
            trimming.set(false);
         }
      }

      //
      return null;
   }
}